Electronic measurement and testing systems use relays and/or switches to route signals. Switching devices used in these systems are required to have a very high off-resistance and a very low on-resistance. MOS analog switches have the disadvantage of non-zero leakage current, high on-resistance and parasitic capacitance.
An example of a prior art microswitch 10 is illustrated in FIGS. 1A and 1B. The basic structure is a micromechanical switch that includes a source contact 14, a drain contact 16, and a gate contact 12. A conductive bridge structure or beam 18 is attached to the source contact 14. As shown in FIG. 1B, the bridge structure 18 overhangs the gate contact 12 and the drain contact 16 and is capable of coming into mechanical and electrical contact with the drain contact 16 when deflected downward. Once in contact with the drain contact 16, the bridge 18 permits current to flow from the source contact 14 to the drain contact 16. An electric field is applied in the space 20 by a voltage on the gate 12. With a sufficiently large field in the space 20, the switch closes and completes the circuit between the source and the drain by deflecting the bridge structure 18 downwardly to contact the drain contact 16.
Switches of this type are disclosed in U.S. Pat. No. 4,674,180 to Zavracky et al., the disclosure of which is incorporated by reference herein. In this device, a specific threshold voltage is required to deflect the bridge structure so that it may contact the drain contact. Once the bridge comes into contact with the drain contact, current flow is established between the source and the drain.
During operation, hysteresis can arise if the voltage required to draw the end of the beam into contact with the drain contact is greater than that required to hold it in contact with the drain. Thus, two modes of operation exist--a hysteretic mode and a non-hysteretic mode. In a hysteretic mode, when the switch is closed, the gap between the beam and the gate is reduced and therefore the gate voltage required to maintain the beam in its downward deflected state is less than the gate voltage required to actuate the switch. To release the beam so that the beam returns to its open state requires a reduction in the gate voltage to a level below not only the gate voltage required to deflect the beam, but also less than the gate voltage required to maintain the beam in its deflected position. A non-hysteretic mode of operation occurs when the switch has a minimum gate actuation voltage approximately equal to the maximum gate release voltage due in part to a longer beam length and larger gate area. Thus there is a particular threshold voltage, above which the beam will be deflected downward, and below which the beam will be released.
Another consideration is that the drain end of the switch may also experience an electrostatic force for high drain/source voltages. Increasing the drain/source voltage above a critical value will cause an unstable operation of the device and may deflect the beam, establishing contact between the drain and the source. This effect is the equivalent of breakdown in a solid state device.
A relay having a contact that is isolated from the beam is disclosed in U.S. Pat. No. 5,638,946 to Zavracky et al., the disclosure of which is incorporated by reference herein. Referring to FIGS. 2A and 2B a micromechanical relay 28 is shown that includes a substrate 30, and a series of contacts mounted on the substrate. The contacts include a source contact 32, a gate contact 34, and a drain contact 36. The drain contact 36 is made up of two separate contacts 37 and 37'. A beam 38 is attached at one end 40 to the source contact 32 and permits the beam to hang over the substrate. The beam is of sufficient length to overhang both the gate contact 34 and the drain contact 36. The beam 38 illustrated in FIGS. 2A and 2B includes an insulative element 42 that joins and electrically insulates the beam body 44 from the beam contact 46. In operation, actuation of the relay permits the beam contact to connect the two separate contacts 37 and 37' of the drain contact 36 and allow current to flow from one separate drain contact to the other. It has been found that the insulator to beam interface can be mechanically weak, and that the insulator may separate from the beam, resulting in failure of the device.